The present invention relates generally to endoprosthesis devices, which are commonly referred to as stents, and more particularly pertains to improvements thereto that provide for a reduced delivery profile, increased vessel scaffolding and in the case of balloon expandable stents, improved stent security.
Stents are generally thin walled tubular-shaped devices composed of complex patterns of interconnecting struts which function to hold open a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for supporting a dissected arterial lining or intimal flap that can occlude a vessel lumen. There are two general classes of stents, balloon expandable stents and spring-like self-expandable stents. Balloon expandable stents are delivered within a vessel lumen by means of a dilatation catheter and are plastically deformed by means of an expandable member, such as an inflation balloon, from a small initial diameter to a larger expanded diameter. Self-expanding stents by contrast are formed as spring elements which are radially compressible about a delivery catheter. A compressed self-expanding stent is typically held in the compressed state by a delivery sheath. Upon delivery to a lesion site, the delivery sheath is retracted allowing the stent to expand.
Either type of stent has advantages and disadvantages. One disadvantage of self expanding stents with respect to balloon expandable stents is the relatively large profile of the self-expanding stent-delivery system. Because, self-expanding stents typically are restrained during delivery by a sheath, the outside diameter of a self-expanding stent-delivery system is greater than that of a comparable balloon expandable stent-delivery system which does not require a delivery sheath. As such, balloon expandable stents may be delivered within smaller vessels than can now be reached with a self-expanding stent.
One disadvantage of balloon expandable stents is the tendency for the stent to slip on the inflation balloon. Balloon expandable stents are typically crimped onto an inflation balloon in such a manner as to provide for a uniform crimp of the stent about the balloon. Typically, such crimps adequately secure the stent to the balloon. However, in certain circumstances, such as when the stent encounters obstacles such as hardened plaque or a flap of tissue partially torn from a vessel wall, stents occasionally slide off of the delivery catheter. It is believed that this slippage problem occurs because the stent forms a smooth continuous interface with the balloon and may not always generate sufficient frictional resistence to remain positioned on the balloon when encountering obstacles within the patient""s vasculature. One approach to solving the slippage problem is to protect the balloon expandable stent with a delivery sheath. However, the use of a delivery sheath negates the reduced profile advantage of balloon expandable stent-delivery systems with respect to self-expanding stent-delivery systems.
Both balloon expandable and self-expandable stents must be able to simultaneously satisfy a number of interrelated mechanical requirements. First, the stent must exhibit sufficient radial or hoop strength in its expanded state to withstand the structural loads, namely radially compressive forces, imposed on the stent by the walls of a vessel. It is advantageous to distribute such loads over as much of the stent as possible and over as much lumen wall as possible. Uniform loading minimizes the possibility of localized crippling of the stent which may induce a general structural failure. In addition, uniform loading tends to minimize injury to the vessel wall. Second, it is desirable for a stent to provide a high degree of scaffolding of the lumen walls, i.e., minimize the gaps between stent struts, in order to prevent prolapse of plaque between the struts and into the lumen. Third, a stent should be sufficiently radiopaque to be readily visible by fluoroscopy procedures. Radiopacity with typical stent materials such as stainless steel and nickel-titanium alloy is generally a function of the stent""s mass, and is in particular a function of the thickness of the stent struts. Fourth, a stent should be longitudinally flexible in order to be delivered through tortuous vessels. Finally, a stent should have a small crimped or initial delivery diameter in order to facilitate advancement through small vessels and must be able to expand to a second larger diameter for implantation within a body lumen. Generally, stents with large expansion ratios are preferred.
A number of approaches have been devised in an effort to address these various requirements. A popular early approach called for the stent to be constructed wholly of wire. The wire is bent, woven and/or coiled to define a generally cylindrical structure in a configuration that has the ability to undergo radial expansion. Wire stents have a number of disadvantages including for example, a substantially constant cross-section which may cause greater or lesser than an ideal amount of material to be concentrated at certain locations along the stent. Wire also has limitations with respect to the shapes into which it may be formed, thus limiting the expansion ratio, coverage area, and strength that can ultimately be attained.
More recent stents have been constructed from drawn tubing. By selectively removing material from tube stock, a stent with complex patterns of interconnecting struts may be formed. At present, there are numerous commercial tube based stents with a variety of strut geometries being marketed throughout the world. Whether a stent is self-expanding or balloon expandable is primarily a function of the stent""s material. Balloon expandable stents and self-expanding stents may have the same geometric pattern with the determining factor being the use of a spring-like or shape memory material for a self-expanding version of the stent or the use of a plastically deformable material for a balloon expandable version of the stent. The selection of a particular pattern has a profound effect on the radial strength, scaffolding, radiopacity, and initial or crimped profile of the resulting stent.
While tube-based stents offer many advantages over the early wire-based designs, there remains room for improvement in the art. In particular, there is a need to reduce the delivery profile of stents, particularly with regard to self-expanding stents, and to increase the security of balloon expandable stents on the inflation balloon. The present invention meets these and other needs.
The present invention provides a stent which may be crimped to a smaller delivery diameter than previously known stent devices for a given metal density. In addition, the stent may positively engage the material of an inflation balloon, in a manner previously thought undesirable in the art, and thereby improve stent security on the balloon. The stent achieves these results while continuing to fulfill all of the mechanical and structural requirements attendant to its function as a stent.
The stent of the present invention can take many forms and preferably is configured to form a tubular member having a lattice structure of struts. The stent has a smaller delivery diameter in which it is preferably mounted on a catheter, and an expanded diameter wherein it is implanted in a body lumen, such as a coronary artery. When the stent is in the smaller delivery diameter, at least some of the struts overlap so that the stent has a very small profile, increased gripping force on the catheter, and is able to achieve a larger expanded diameter. The struts can overlap circumferentially or along the longitudinal axis of the stent, or both.
In one embodiment, the stent of the present invention includes generally a plurality of cylindrical rings that are interconnected by a plurality of links. When viewed in isolation, each cylindrical ring is generally formed as a pattern of alternating U-shaped portions which may be thought of as comprising a plurality of peaks and valleys, where a peak represents the apex of the U-shaped portion and a valley represents the space between the struts which form the open end of the U. The stent""s advantages are achieved by circumferentially offsetting the valleys of each cylindrical ring from those of adjacent cylindrical rings and by nesting the rings such that the peaks of each adjacent cylindrical ring are centered midway within the valleys of each preceding ring. The U-shaped portions are formed from struts which are configured with an angled step midway along their length. The angled step essentially divides each strut into an upper portion and a lower portion. Thus, when the stent is crimped or compressed to its initial delivery diameter, the lower portion of each strut slides underneath the upper portion of each strut in a preceding nested ring. Thus, when compressed, the stent forms a pattern of nested cylindrical rings with overlapping struts. The overlapping struts allow the stent to be crimped to an initial delivery diameter smaller than that of prior art non-overlapping strut designs for a given metal density. Therefore, the profiles of both balloon expandable and self-expandable stent-delivery systems may be reduced with the present invention stent. Reduced delivery system profiles allow smaller vessels to be treated with the present invention stent than can be reached with prior art stents. In addition, in balloon expandable embodiments of the stent, the overlapping strut configuration allows the struts to engage or pinch the balloon material, thereby increasing stent security on the balloon.
Each of the cylindrical rings making up the stent has a proximal ed and a distal end and a cylindrical plane defined by a cylindrical outer wall surface that extends circumferentially between the proximal end and the distal end of the cylindrical ring. The cylindrical rings are interconnected by at least one longitudinal connecting link which attaches one cylindrical ring to an adjacent cylindrical ring. In addition, each connecting link may be circumferentially offset from the previous connecting link in a preceding ring. Circumferentially offsetting the links increases the longitudinal flexibility of the stent. The connecting links are positioned substantially within the cylindrical plane of the outer wall surface of the cylindrical rings.
Each connecting link is about one half the length of the struts which form each U-shaped portion. The connecting links are positioned so that they are within the curved part of a U-shaped portion and connect the apex of one U-shaped portion to the apex of a preceding U-shaped portion, positioning the preceding U-shaped portion about half way within the valley of the succeeding U-shaped portion. Typically, balloon expandable embodiments of the stent may be formed from a plastically deformable metal alloy such as stainless steel and similar materials. Self-expanding embodiments may be formed from shape memory alloys or superelastic alloys which expand upon undergoing a martensite to austenite phase change. Such a phase change is typically initiated by increasing the temperature of the alloy above a predetermined transition temperature, or in the case of alloys which exhibit stress induced martensite phase changes, upon relieving an imposed stress.
The stent may be formed from a tube by laser cutting the pattern of cylindrical rings and connecting links in the tube. The stent also may be formed by laser cutting a flat metal sheet in the form of the cylindrical rings and links, and then rolling the pattern into the shape of the cylindrical stent and providing a longitudinal weld to form the stent.
These and other features and advantages of the present invention will become apparent from the following detailed description, which when taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention.